I Need Advice For This Job Interview?

I've never had a job before, last year I got one job application and failed it. They told me they didn't currently have any positions, which I know was bull (it was at the same place but I was never actually offered an interview I just called up and said I applied and was wondering if they could interview me, this time they called me though). I was kind of stomped on the questions. But it's at six flags so maybe you guys with experience could give me a few tips. I'm in the Marines DEP, and leaving for bootcamp August 8th, but I was told never to tell any potential employer about any other job. So when they ask me, why I want to work at six flags, what should I tell them? And also she asked me what do I do if some visitors to the park begin arguing or something like that. What should I do? Thanks in advance.